Simplify the condition under which we flush the compaction queue.

This code is equivalent to what we had before.  Given the constants
in the code, we were only flushing based on the queue size when one
of those two conditions was true:
  - The size of the queue is bigger than 100
  - The size of the queue is bigger than 18000
Which obviously can be simplified as checking only against 100.

Thanks to Brent Evans for pointing this out.
Swap the order in which conditions are evaluated to flush the queue.

The "size > threshold" condition is almost always going to be the
condition that triggers a flush.